INSERTS CAST IN GIRDERS

Fig. 2.14 Intermediate diaphragm composed of structural steel shapes.

4 6 H I G H W A Y B R I D G E S U P E R S T R U C T U R E S

A novel approach to providing stiff intermediate diaphragms in girder bridges that has been successfully used on bridges constructed for the lumber industry in the State of Washington is shown in Fig. 2.14 (Ref. 37).

These diaphragms consist of structural steel sections installed by connect-ing them to small pieces of steel cast into the precast beams. Diaphragms of this type can be installed much more rapidly than the more conventional cast-in-place concrete diaphragms. They do, however, require mainte-nance as do bridges incorporating steel beams and trusses.

2 . 4 Decks for Girder Bridges

In the design of the a girder bridge, a distinction must be made between bridges which have flexurally-stiff intermediate diaphragms and those which do not. As previously pointed out, the individual bridge girders of normal configuration (i.e. I-shaped or T-shaped beams) have virtually no torsional stiffness and hence do not afford rotational restraint to the decks which they support. Because the decks themselves are rela-tively flexible in comparison to the girders, differential girder live load deflection must be considered for bridges which do not have adequate intermediate diaphragms.

In view of this, it is recommended that the decks of girder bridges containing more than two girders be designed using the empirical relation-ships of the AASHTO Specification in cases where the girders are torsion-ally flexible and well designed flexurtorsion-ally stiff intermediate diaphragms are not provided between them.

In the case of multi-girder bridges incorporating girders which are tor-sionally flexible but connected by well designed flexurally-stiff inter-mediate diaphragms, it is recommended the decks be designed as being continuous over the supporting girders without differential settlement bet-ween supports. The analysis should be made on an elastic basis using the

(Ref. 17) or Homberg (Ref. 18) charts or other sophisticated methods of analysis. The girders should not be considered as providing rotational restraint to the deck.

As a means of illustrating the effects of differential girder deflection on the stresses in the deck of a bridge incorporating precast I-girders, the results of a finite element analysis for the cross section of Fig. 2.1 are summarized in Fig. 2.15. The analysis was made for the bridge having a simple 120 feet and having one intermediate diaphragm at

Two concentrated loads of 100 kips were applied to the bridge. The loads were applied over the middle girder 30 feet from each end of the bridge as

GIRDER BRIDGES 4 7

100 I

100 I

D I A P H R A G M

100

4 3 2 1

4 = -7.47 x RADIANS

= 0 RADIANS

Fig. 2.15 Study of secondary stresses in the deck of a bridge; (a) loading arrangement, , (b) at of application of a 100 kip load, and (c) elastic curve of the deck between points 3 and 4.

H I G H W A Y B R I D G E S U P E R S T R U C T U R E S

shown in Fig. 2.15(a). Under this condition of loading, the maximum stress in the slab occurs at the point of application of the load and is of the order of 320 psi. When it is realized that the 100 kip load is of the order of five times the design wheel load (including impact) used with HS

loading it becomes apparent the live load stresses due to differential girder deflection in structures of this type are of secondary magnitude and can safely be ignored in practice providing the design criteria being employed reasonably reflects the actual wheel loads to be imposed on the structure.

As pointed out above, bridge superstructures which contain two torsionally flexible longitudinal girders constitute a special case. The decks for bridges of this type (see Fig. 2.7) should be designed as simply sup ported spans because the torsionally flexible girders provide virtually no joint restraint at all. The designer has the option of designing decks of this type with the influence charts of (Ref. 17) or Homberg (Ref. 18) or, of course, with the empirical coefficients of the AASHTO Specification.

The use of the influence charts will yield more rational results.

When torsionally stiff girders are used, it is recommended that special methods of analysis similar to those discussed in Section 4.4 be employed.

2.5 Continuity

Prestressed concrete girder bridges utilizing cast-in-place T-beams are not a commonly used form of construction. Cast-in-place reinforced concrete T-beam bridges are rather frequently used and it is not uncommon for them to be made continuous over three or more supports. Concrete T-beams have found greatest use in bridges of medium span length and this probably accounts for the rather infrequent use of this shape with prestressing. The design of a T-beam bridge using prestressed reinforcement is straightfor-ward and only the computation secondary moment resulting from the prestressing is a factor that is markedly different from the design proce-dures that must be followed with the two different materials.

Continuity has often been established in bridges incorporating precast prestressed concrete beams. This is normally accomplished through the provision of reinforcing steel in the cast-in-place deck which is placed over the precast girders. In this mode of construction the major portion of the dead load is carried by the precast beams acting as simple beams. The dead load of railings, sidewalks, wearing surfaces and other superimposed loads are carried by the continuous structure, as are the live loads. In the design of bridges type, positive moments may occur at the interior

due to the effects of temperature as well as creep and shrinkage of the

GIRDER BRIDGES

concrete. Special consideration should be given to these effects. In addi-tion, due to the special nature of the stresses in the prestressed concrete girders and cast-in-place diaphragms for bridges of this type, certain devia-tions from the usual allowable concrete stresses at service and design loads seem to be appropriate. These special requirements appear in reference 38.

2.6 Overhanging Beams

Occasionally the bridge designer is faced with providing a high, long, channel span over a waterway in order to provide horizontal and vertical clearance for navigation. A traditional method of accomplishing this in

steel bridges has been with the use of a span suspended from two overhang-ing beams. This mode of framoverhang-ing, which is illustrated in Fig. 2.16, presents certain difficulties when done with i-shaped prestressed concrete girders.

Most of these difficulties are in the form of construction sequences and methods that must be followed. The effort required to design a bridge of this type is aggravated by the need to establish the construction sequence that must be followed. These difficulties can generally be eliminated or minimized by the use of a cast-in-place box girder section for the overhang-ing beams, in lieu of the use of I-shaped girders.

ABUT. PIER PIER ABUT.

66’ 1 4 5 ’ 6 6 130

I - I

-Fig. 2.16 Elevation of a bridge with two overhanging beam spans and a suspended span.

Problems which have been encountered in the use of I-shaped stressed concrete girders in bridges with overhanging beams include the following:

1. Longitudinal prestressing of the girders frequently has to be done in several stages, as the construction progresses, in order to control initial concrete stresses in the structure.

2. Temporary prestressing tendons frequently must be used to control stresses in the girders at certain stages in the construction.

3. The sequence of placing concrete in the end diaphragms,

SO HIGHWAY BRIDGE SUPERSTRUCTURES

mediate diaphragms and deck slabs must take into account the effects of elastic and non-elastic shortening of the concrete.

4 . Elastic stability of the girders (buckling) must be given consideration.

5. Erection procedures that can be used with the suspended spans may be restricted due to the stresses that could be imposed on the over-hanging beams.